1. Field of the Invention
The invention relates to lock-up control apparatus in an automatic transmission of a vehicle having a torque converter between an engine and a transmission mechanism.
2. Description of Related Art
Japanese Patent Laid-Open Publication No. HEI 7-279700 discloses a control apparatus for an automatic transmission of an automobile with a torque converter between the engine and transmission, which control apparatus determines a coasting state, in which the engine is driven by the transmission, based on an index related to engine torque, and when the coasting state is detected, the engine speed is increased when the lock-up clutch is engaged. With this arrangement, it is possible to engage the lock-up clutch to secure the engine brake even in a low speed coasting state.
In the above-described control apparatus, as a method for detecting the coasting state, an accelerator OFF is judged by the index related to the engine torque, i.e., by a signal from a throttle sensor.
However, there is a problem, in terms of detection precision, in judging the coasting state using a signal from the throttle sensor or a signal from the accelerator as an index related to the engine torque, or a signal directly from the engine. Even if the coasting state is judged only when the accelerator opening is 0, in actual operation, the coasting state is established even when the accelerator opening is other than 0 in some cases.
For example, a coasting state may be established when a vehicle runs on a steep downhill, the accelerator pedal is slightly depressed and the vehicle speed is increasing. In such a case, although the vehicle is actually in a the coasting state in which the engine is driven by the transmission, since the accelerator pedal is depressed, the coasting state can not be detected, and the torque-up control can not be effected. Therefore, the lock-up clutch can not be reliably engaged, and the engine brake can not be applied in some cases.
Accordingly, it is an object of the present invention to provide a lock-up control apparatus in an automatic transmission capable of reliably engaging a lock-up clutch in any vehicle state, and capable of applying an engine brake.
According to a first aspect, the present invention provides a lock-up control apparatus in an automatic transmission having a torque converter between the engine and transmission, in which the torque converter includes a lock-up clutch capable of directly coupling an output shaft of the engine and an input shaft of the transmission. The lock-up control apparatus includes a lock-up clutch drive capable of engaging the lock-up clutch by hydraulic pressure in a state in which the engine speed is lower than the speed of the input shaft of the transmission mechanism. The lock-up control apparatus further includes speed difference detecting means for detecting a difference between the rotational speeds of the engine and the transmission input shaft; and engine speed control means for increasing the engine speed if the difference in rotational speeds detected by the speed difference detecting means exceeds a predetermined limit when the lock-up clutch is engaged.
Accordingly, if the rotational speed difference between the engine and the transmission input shaft detected by the speed difference detecting means exceeds the predetermined limit, when the lock-up clutch is engaged, the engine speed is increased to reduce the difference between the engine speed and the speed of the input shaft of the transmission. At that time, the speed difference detected by the speed difference detecting means can precisely indicate an actual coasting state of the vehicle. Thus, in any vehicle state, e.g., even when the throttle is slightly open on a steep downhill, it is possible to reliably engage the lock-up clutch, and to secure the engine brake. With such a control, the engine speed approaches the input shaft speed. In this manner, when the lock-up clutch is engaged, the speed difference between the engine and the transmission input shaft, to be absorbed by the lock-up clutch, is reduced. Therefore, even with the engaging hydraulic pressure of the lock-up clutch smoothly increased, the engagement can be completed within a short time, and the gear shift time can be shortened. The shift shock due to the difference in speeds between the engine and the input shaft of the transmission can also be prevented.
In a preferred embodiment, the engine speed control means includes engine control means for directly controlling the engine so that it becomes easy to control the engine speed. According to another preferred embodiment, the engine speed control means further increases output torque of the engine as the difference between the engine speed and the speed of the input shaft of the transmission becomes greater.
In the above-described preferred embodiment, it is possible to cause the actual speed difference to converge within a predetermined speed difference, irrespective of the magnitude of the speed difference. Further, if the speed difference is reduced, the engine torque can be set smaller and, thus, it is possible to prevent the engine speed from exceeding the input shaft speed, so as not to give the driver an unpleasant sensation. With such a control, when the lock-up clutch is released during gear shifting, even if the input shaft speed changes, the torque can be increased without time lag.
According to still another preferred embodiment, when the difference between the engine speed and the speed of the input shaft of the transmission remains unchanged, the engine speed control means reduces the engine output torque as the speed of the input shaft of the transmission mechanism increases. In this preferred form, it is possible to provide the desired control while taking the characteristics of the torque converter into consideration.
According to yet another preferred embodiment, when the difference between the engine speed and the speed of the input shaft of the transmission converges within a predetermined range, the engine speed control means maintains the output torque constant for a predetermined time (e.g., until the operation of engaging the lock-up clutch is completed). By maintaining the output torque constant, the engine speed can be maintained constant, and the operation of engaging the lock-up clutch can be smoothly effected.
According to yet another preferred embodiment of the invention, when the difference between the engine speed and the speed of the input shaft of the transmission converges within the predetermined range, if the output torque of the engine is positive, the engine speed control means maintains the output torque at 0. With this preferred form, when the output torque of the engine is positive, the engine drives the transmission mechanism. Therefore, by maintaining the output torque at 0, it is possible to prevent the engine speed from exceeding the input shaft speed, and to prevent the speed difference therebetween from increasing, whereby the lock-up clutch can be engaged smoothly.
According to another preferred embodiment, when the difference between the engine speed and the speed of the input shaft of the transmission mechanism converges within the predetermined range, if the output torque of the engine is negative, the engine speed control means maintains the output torque at that obtained when the difference converged within the predetermined range. In this preferred embodiment, when the output torque of the engine is negative, by maintaining the output torque at that obtained when the difference converged within the predetermined range, it is possible to maintain the difference between the engine speed and the input shaft speed within the predetermined range, and the lock-up clutch can be engaged smoothly.
According to still another preferred embodiment, when the lock-up clutch is engaged, the lock-up clutch drive sweeps up the engaging hydraulic pressure at a predetermined gradient to execute engagement. The sweep up is continued for a predetermined time after the difference between the engine speed and the speed of the input shaft of the transmission converges within the predetermined range and, then, the engagement of the lock-up clutch is maintained by a holding hydraulic pressure. With this preferred embodiment, after the difference between the engine speed and the speed of the input shaft of the transmission converges within the predetermined range, the sweep up is continued for a predetermined time so that the lock-up clutch can be engaged smoothly.
According to still another preferred embodiment, when the lock-up clutch is engaged, the lock-up clutch drive (driving means) sweeps up engagement hydraulic pressure at a predetermined gradient to execute the engaging operation and, at the instant when the difference between the engine speed and the input shaft speed converges within the predetermined range, the lock-up clutch drive immediately holds the engagement by application of a holding hydraulic pressure. With this preferred embodiment, the engaging operation of the lock-up clutch can be completed within a short time, thereby shortening the gear shift time.
According to yet another preferred embodiment, the engine speed control means maintains the output torque constant for a predetermined time after the engaging operation of the lock-up clutch, by the lock-up clutch driving means, is completed. With this preferred embodiment, the engine speed control means maintains the output torque constant for a predetermined time after the engaging operation of the lock-up clutch by the lock-up clutch driving means is completed. Therefore, the delay of the engaging operation of the lock-up clutch due to the response delay of the hydraulic pressure can be compensated, and the operation of engaging the lock-up clutch can be reliably completed.
According to a further preferred embodiment, the engine speed control means maintains the output torque constant for a predetermined time after the engaging operation of engaging the lock-up clutch by the lock-up clutch driving means is completed and, then, the engine speed control means sweeps down at a predetermined time. In this manner, it is possible to prevent the output torque of the engine from being abruptly changed, and to prevent a shift shock.
According to yet another preferred embodiment, when the output torque is increased, the output torque is limited within a range of a predetermined threshold value. Therefore, the engine speed is prevented from being abruptly changed, and the control is facilitated.
The difference between the engine speed and the speed of the input shaft of the transmission mechanism may be taken as a speed value whereby the difference between the engine speed and the speed of the input shaft of the transmission is easily obtained. Alternatively, the difference between the engine speed and the input shaft speed may be taken as a ratio (xe2x80x9cspeed ratioxe2x80x9d) between the engine speed and the input shaft speed.
According to still another preferred embodiment, when shifting from a first gear stage where the lock-up clutch is engaged to a second gear stage, the lock-up clutch driving means temporarily releases the lock-up clutch, and when the lock-up clutch is again engaged upon shifting to the second gear stage, the engine speed control means controls the engine speed so that the difference between the engine speed and the input shaft speed converges within the predetermined range. With this preferred embodiment, the engine brake is secured and the lock-up clutch is smoothly engaged without shift shock.
According to yet another preferred embodiment, when shifting from a first gear stage where the lock-up clutch is not engaged to a second gear shift stage, the engine speed control means controls the engine speed so that the difference between the engine speed and the input shaft speed converges within the predetermined range. In this manner the invention can be applied to secure the engine brake and to provide smooth engagement of the lock-up clutch without shift shock.
According to another preferred embodiment, when the lock-up is changed from its disengaged state at a given gear stage to its engaged state at the same gear stage, the engine speed control means controls the engine speed so that the difference between the engine speed and the input shaft speed converges within the predetermined range. In this manner also, the invention can be applied to secure the engine brake and to realize smooth engagement of the lock-up clutch without shift shock.
According to yet another preferred embodiment, the engine is provided with a fuel-cut device and the lock-up control apparatus further comprises fuel cut control means for discontinuing a fuel cut control carried out in the first gear shift stage in which the lock-up clutch is engaged, when the lock-up clutch is temporarily released by the lock-up clutch driving means. With this preferred embodiment, in the first gear stage, even if a fuel cut control has already been executed, it is possible to prevent the engine speed from being lowered abruptly when the lock-up clutch is released. Further, it is possible to swiftly bring the engine speed within the predetermined range by the engine speed control means, and the lock-up clutch can be engaged without a time lag. A fuel-cut device and control thereof are described, for example, in U.S. Pat. No. 5,651,752 entitled xe2x80x9cLock-Up Control System for Automatic Transmissionxe2x80x9d, the teachings of which are incorporated herein by reference.
According to a further preferred embodiment, the engaging of the lock-up clutch by the lock-up clutch driving means is carried out simultaneously with the control of the engine speed by the engine speed control means. Therefore, the time lapsed from the instant of output of a command to start the operation of engaging the lock-up clutch to the instant when the operation of engaging the lock-up clutch is actually started, and a time required for converging the speed difference by the engine speed control means can be matched. Therefore, as compared with a case in which the lock-up clutch is engaged after the engine speed is controlled, the lock-up clutch can be more swiftly engaged without time lag.
According to another preferred embodiment, the lock-up control apparatus further comprises request torque calculating means for calculating engine torque requested by a driver by operation of an accelerator, and engine control releasing means for releasing the control of the engine by the engine control means when the engine torque calculated by the request torque calculating means exceeds output torque of the engine under control of the engine control means. In this preferred embodiment, upon engaging the lock-up clutch, when the coasting state is judged based on an index related to the engine torque, e.g., based on the throttle opening as an index, when the throttle opening becomes other than 0, the torque increase control is released. For example, when the vehicle is running on a steep downhill and its accelerator pedal is slightly depressed and the throttle opening thereby becomes other than 0, even if the actual vehicle state is in the coasting state although the accelerator pedal is depressed and the throttle opening becomes other than 0, the torque increase control is released. If the torque increase control is released, the output torque of the engine is set to a torque corresponding to the accelerator operation. But since the accelerator pedal is depressed only slightly, the output torque of the engine is smaller than the output torque by the torque increase control. Therefore, the lock-up clutch can not be engaged reliably in some cases. However, according to this invention, the engine control by the engine control means is released when the output torque requested by the driver by operation of the accelerator exceeds the output torque which is under torque increase control. Therefore, even if the control is released, the output torque is greater than the output torque which is under torque increase control, and the lock-up clutch can be reliably engaged.
In a second aspect of the invention, there is provided a lock-up control apparatus in an automatic transmission having a torque converter between an engine and a transmission mechanism, in which the torque converter includes a lock-up clutch capable of directly coupling an output shaft of the engine with an input shaft of the transmission mechanism. The lock-up control apparatus includes lock-up clutch driving means capable of engaging the lock-up clutch by hydraulic pressure in a state in which the engine speed is lower than the speed of the input shaft of the transmission mechanism. The lock-up control apparatus further includes speed difference detecting means for detecting a difference between the engine speed and the input shaft speed, and speed difference control means for reducing the speed difference if the speed difference detected by the speed difference detecting means exceeds a predetermined range when the lock-up clutch is engaged.
With the second aspect, if the speed difference between the engine speed and the input shaft speed detected by the speed difference detecting means exceeds the predetermined range, when the lock-up clutch is engaged, the speed difference control means, such as the control unit, the engine control system and the lock-up control program together provide control whereby the difference between the engine speed and the speed of the input shaft of the transmission mechanism is reduced. At that time, the speed difference detected by the speed difference detecting means can precisely detect the actual coasting state of the vehicle. Thus, in any of the states of the vehicle, e.g., even when the throttle is slightly depressed on a steep downhill, it is possible to reliably engage the lock-up clutch, and to secure the engine brake. With such a control, the engine speed approaches the input shaft speed. With this, when the lock-up clutch is engaged, the speed difference between the engine speed and the input shaft speed to be absorbed by engagement of the lock-up clutch is reduced. Therefore, even if the engaging hydraulic pressure of the lock-up clutch is smoothly increased, the engagement can be completed within a short time, and the gear shift time can be shortened. The shift shock caused due to the difference in speeds between the engine and the transmission input shaft can also be prevented.